1. Field of the Invention
This invention relates to improvements in aircraft navigational instruments, and more particularly to improvements in aircraft ground speed indicators and circuits for developing ground speed indicating signals.
2. Description of the Prior Art
One of the important navigational aircraft instruments is a ground speed indicator. Ground speed is useful in deriving estimated time of arrival, in calculating speed of head or tail winds, in evaluating aircraft performance, and is often merely of general interest.
Although ground speed is useful, it has heretofore been a measurement which is relatively difficult to obtain. Generally unsatisfactory results have been achieved with prior art analog techniques particularly when using area navigation equipment to travel along paths which do not cross VOR/DME stations. Such techniques typically derive ground speed through the use of an analog differentiator which operates on an appropriate DME signal. Ordinarily, however, such techniques provide correct ground speed only when the course of the aircraft passes directly over the DME station. When the aircraft course is between way points not coinciding with the DME station, the ground speed signal derived typically is only a component of the true ground speed, generally extending in the direction of the DME station. Therefore it can be seen that if the aircraft crosses a radial at right angles to the station, the ground speed may be especially erroneous. The ground speed derived by such analog techniques, therefore, can be used only as a rough estimate, at best.
The prior art includes digital techniques for deriving ground speed, as well. However, many digital systems derive ground speed in a fashion similar to that of the analog techniques described above, providing only a partial component or a rough ground speed estimate. Other sophisticated digital techniques provide true on-course ground speed, but require complicated circuitry, often using "dead reckoning" or "estimated aircraft" positioning techniques.
One common prior art problem is meeting the strict requirements of ground speed deriving circuits. Commonly, DME and VOR signals from which ground speed mesurement is made are highly noisy. The circuitry to handle the signals, nevertheless, has to be linear over a wide range of signal magnitudes. This large linearity requirement is aggravated in great part by the range of the distances over which the DME and VOR signals must be detected and processed, typically over 300 nautical miles. Thus, from a position at the fringes of reception of the DME or VOR signals, having an extremely high noise content, to a location directly over the DME or VOR station, the signal to be processed varies perhaps in orders of magnitude. Consequently, the circuitry may detect large magnitudes of noise which itself may be orders of magnitude larger than the actual velocity/track angle signal content sought. Often times, in fact, computer processing techniques are used to separate the signal from the noise. This large linearity range causes additional problems, especially in the initialization or reinitialization of the ground speed deriving circuit, after, for example, a turn or other disturbing event.